1. Field of the Invention
The present invention relates to cellulosic particles, a suspension of cellulosic particles and a process for the production of a suspension of cellulosic particles, whereby the cellulosic material is never dried between the dissolution of the cellulose and the disintegration of the suspended cellulose fibers.
2. Description of Related Art
Dry cellulose powders are commercially available in various sizes and are used in a number of different applications, such as for example as auxiliary filter materials, additives and auxiliary materials in food stuffs and pharmaceutical products, chromatography materials as well as in the form of additives in the building material trade. The greater part, which is extracted from pulp, wood or one-year plants, is accounted for by fibrous cellulose-I-powder. In this respect the lower limit of the fiber length of this fibrous cellulose powder is limited to 10-20 μm. In the upper range fiber lengths are used in the mm range whereby there is already some overlapping with short-cut fibers.
In smaller quantities cellulose-II-powder can also be found whereby spherical powder can be found in addition to fibrous powder. These powders are mainly made by the precipitation of dissolved cellulose in suitable precipitants. Spherical cellulose powders in the size range below 10 μm can only be made with much more effort and are thus difficult to find on the market.
For example WO 02/57319 describes the production of cellulose pearls using the NMMO-process whereby large amounts of various additives are added to the cellulose solution prior to forming, such as for example titanium dioxide or barium sulphate as well as materials which provoke ion exchange. The products obtained can be used as ion exchangers or catalysts.
The production of a titanium oxide suitable as an ion exchange material, for example for waste water purification, is described in U.S. Pat. No. 6,919,029. Particularly high absorption capacities and speeds are attained with this material by means of the fact that the titanium oxide material is activated by a special treatment on the surface. This titanium oxide material can be described as a “substoichiometric titanium oxide”. This means that the ratio of the oxygen atoms to the titanium atoms in the material is smaller than 2. For a more detailed description of this surface activation, reference is made to the description in U.S. Pat. No. 6,919,029.
Other possibilities for the production of particular functionalized titanium oxides can be found in the so-called “doping” of titanium oxide with iron and sulphur atoms. These compounds display a photocatalytic activity.
Cellulosic materials in the mm range are likewise being given greater attention recently. In this respect one can differentiate between the rigid crystalline Whiskers (de Souza Lima, M. M. and R. Borsali, Macromolecular Rapid Communications, 2004. 25: p. 771-787) and the flexible MFC (Microfibrillated Cellulose) (Herrick, F. W. et al, Journal of Applied Polymer Science; Applied Polymer Symposium, 1983. 37: p. 797-813) and also (Turbak, A. F. F. W. Snyder and K. R. Sandberg, Journal of Applied Polymer Science; Applied Polymer Symposium, 1983. 37: p. 815-827). Both particle types are smaller in the size range of around 1 μm and are in the form of suspensions or gels with only a slight cellulose content due to their production process. Production is done largely via one or several mechanical disintegration steps (ultrasound, homogenizer, etc.) in combination with a strong degradation of the cellulosic starting material via enzymes or strong acids.
Likewise, cryo processes are described in the literature with the help of liquid nitrogen to release micro-fibrils of cellulosic materials (Chakraborty, A. M. Sain, and M. Kortschot, Holzforschung, 2005. 59: p. 102-107).
An alternative method for the production of cellulose nano-fibers is the electro spinning process (Kulpinski, P., Journal of Applied Polymer Science, 2005. 98 (4): p. 1855-1859) which also demands a great deal of effort.
The main field of application for these nano-structured materials is currently above all the reinforcement of compound materials (Favier, V., H. Chanzy and J. Y. Cavaillé, Macromolecules, 1999. 28: p. 6365-6357).
In the literature, films or membranes are described as another special application of the cellulose particles described above. Often the cellulosic materials used in combination with other substances and/or the production of the films demands a great deal of effort. Examples can be found in (Fendler, A., et al. Characterization of barrier properties of composites of HDPE and purified cellulose fibers. Cellulose, 2007. In press. Doi. 10.1007/s10570-007-9136-x), Liu, H. and Y.-L Hsieh, Ultrafine Fibrous Cellulose Membranes from the Electrospinning of Cellulose Acetate. Journal of Polymer Science: part B: Polymer Physics, 2002.40:p. 2119-2129) or (Sanchez-Garcia, M. D. E. Gimenez and J. M. Lagaron, Morphology and barrier properties of solvent cast composites of thermoplastic biopolymers and purified cellulose fibers, Carbohydrate Polymers, 2007 in press. doi: 10.1016/j.carbpo1.2007.05.041).